1. Field of the Invention
The present invention relates to a switching power supply, and particularly to a capacitor charging apparatus which generates high voltage by charging a capacitor.
2. Description of the Related Art
For the purpose of generating higher voltage than the input voltage, step-up switching power supplies are widely used in various electronic devices. Such a step-up switching regulator includes a switching element and a transformer. With such an arrangement, the switching element is turned on and off in a time shared manner so as to generate back electromotive force in the transformer, thereby charging an output capacitor with the current that flows through the secondary coil of the transformer. Thus, the input voltage is boosted and output.
The input voltage is applied to one terminal of the primary coil of the transformer, and the other terminal is connected to the switching element. The voltage applied to one terminal of the secondary coil of the transformer is set to a fixed voltage, and the other terminal thereof is connected to the output capacitor via a rectifier diode.
With such a switching regulator, when the switching element (switching transistor) is turned on, current flows through the primary line of the transformer, thereby storing energy in the transformer. Subsequently, when the switching transistor is turned off, on the secondary side of the transformer, the energy stored in the transformer is transferred to the output capacitor via the rectifier diode as a charging current, thereby charging the output capacitor. The output capacitor is charged by repeatedly performing the ON/OFF operation of the switching transistor, thereby raising the output voltage.
For example, Patent documents 1 through 3 disclose control circuits for a self-exciting capacitor charging apparatus, which monitor the state of the primary line or the secondary line of a transformer, and control the ON/OFF operation of a switching transistor according to the state thus monitored.
[Patent Document 1]
Japanese Patent Application Laid Open No. 2003-79147
[Patent Document 2]
U.S. Pat. No. 6,518,733
[Patent Document 3]
U.S. Pat. No. 6,636,021
In some cases, the capacitor charging circuit controls the circuit operation according to the output voltage that occurs at the output capacitor. For example, judgment of whether or not an output voltage sufficient to drive the load has been generated, i.e., full-charge detection, is made by monitoring the output voltage. For example, with an arrangement disclosed in the aforementioned Patent document 1, the output voltage is indirectly monitored by monitoring the voltage across the primary coil of the transformer, thereby detecting full-charge.
However, the relation between the voltage across the primary coil and the output voltage changes according to the transformer winding ratio and so forth, leading to difficulty in detecting the output voltage with high precision. As a result, in some cases, there is a problem in that a driving voltage sufficient to drive the load cannot be obtained, or a problem in that an overcharge state occurs in which a charging operation is performed that exceeds the necessary voltage, leading to excessive power consumption.
Furthermore, in a case of directly monitoring the output voltage that occurs at the output capacitor after the output voltage is a divided by resistor, there is a need to employ a high-voltage resistor element. It is difficult to provide such a high-voltage resistor element within an LSI in the form of a built-in element. That is to say, there is a need to provide such a high-voltage resistor element in the form of a chip element, leading to an increased number of circuit components and an increased packaging area. Furthermore, in some cases, there is a need to provide a reverse diode so as to prevent the charge stored in the output capacitor from being discharged to the ground.